Control circuits and methods for controlling switching devices

ABSTRACT

A control circuit for controlling a switching device having a first terminal, a second terminal, and a control terminal is disclosed. The control circuit includes a first diode for coupling to the first terminal of the switching device, a second diode for coupling to the second terminal of the switching device, a first transistor for coupling to the control terminal of the switching device, and a second transistor coupled to the second diode. The first transistor is coupled to the first diode. The control circuit is configured to allow current flow in only one direction between the first and second terminals of the switching device.

FIELD

The present disclosure relates to control circuits and methods forcontrolling switching devices.

BACKGROUND

A variety of controllers are known for controlling switching devices,including field effect transistors (FETs). For example, control circuitsare known for ORing FETs, polarity protection FETs, and synchronousrectifiers incorporated into various applications, such as powersupplies.

In particular, three controllers for MOSFETs are shown in FIGS. 1-3. Asshown in FIG. 1, a controller 100 is connected to control a MOSFET Qhaving a source terminal, a gate terminal, and a drain terminal. Thecontroller 100 includes a bipolar junction transistor Q1 connected tothe source terminal and the gate terminal of the MOSFET Q and a diode D1connected to the drain terminal of the MOSFET Q. The control circuit 100also includes two resistors R1, R2. As shown in FIG. 2, a controller 200is connected to a MOSFET Q having a source terminal, a gate terminal,and a drain terminal. The controller 200 includes two bipolar junctiontransistors Q1, Q2 and resistors R1, R2. The transistor Q1 is connectedto the source terminal of the MOSFET Q, and the transistor Q2 isconnected to the drain terminal of the MOSFET Q. As shown in FIG. 3, acontrol circuit 300 is connected to control a MOSFET Q having a sourceterminal, a gate terminal, and a drain terminal. The control circuit 300includes two bipolar junction transistors Q1, Q2. The transistor Q1 isconnected to the source terminal of the MOSFET Q, and the transistor Q2is connected to the drain terminal of the MOSFET Q. The orientation ofthe transistor Q2 in FIG. 3 is different than the orientation of thetransistor Q2 in FIG. 2. The control circuit 300 also includes tworesistors R1, R2. Each controller allows current to pass in onedirection, while blocking current in a second direction.

While the control circuits discussed above are suitable for theirintended purpose, the present inventors have understood a need for animproved control circuit for switching devices.

SUMMARY

According to one aspect of the present disclosure, a control circuit forcontrolling a rectifier having a first terminal, a second terminal, anda control terminal is disclosed. The control circuit includes a firstdiode for coupling to the first terminal of the rectifier, a seconddiode for coupling to the second terminal of the rectifier, a firsttransistor for coupling to the control terminal of the synchronousrectifier, a second transistor coupled to the second diode, and a thirddiode coupled between the first terminal of the first transistor and thesecond terminal of the first transistor to limit saturation of the firsttransistor. The first transistor is coupled to the first diode. Thecontrol circuit is configured to allow current flow in only onedirection between the first and second terminals of the switchingdevice.

According to another aspect of the present disclosure, a control circuitfor controlling a rectifier having a first terminal, a second terminal,and a control terminal is disclosed. The control circuit includes afirst diode for coupling to the first terminal of the rectifier; asecond diode for coupling to the second terminal of the rectifier, afirst transistor coupled to the first diode and for coupling to thecontrol terminal of the rectifier, a second transistor coupled to thesecond diode and an auxiliary circuit coupled to the first transistorfor adjusting a switching time of the rectifier.

According to one aspect of the present disclosure, a power supply forsupplying a current to an output is disclosed. The power supply includesa rectifier having a first terminal, a second terminal, and a controlterminal, and a control circuit coupled to the synchronous rectifier toallow current flow in only one direction through the rectifier. Thecontrol circuit includes a first transistor having a first terminal anda second terminal, a second transistor coupled to the second terminal ofthe rectifier, and a third diode coupled between a first terminal of thefirst transistor and a second terminal of the first transistor to limitsaturation of the first transistor. The first transistor is coupled tothe first terminal of the rectifier and the control terminal of therectifier.

According to one aspect of the present disclosure, a power supply forproviding a current to a load is disclosed. The power supply includes afirst synchronous rectifier, a second synchronous rectifier having afirst terminal, a second terminal, and a control terminal, and a controlcircuit for controlling the conduction of the second synchronousrectifier. The control circuit includes a first diode coupled to thefirst terminal of the second synchronous rectifier, a second diodecoupled to the second terminal of the second synchronous rectifier, afirst transistor coupled to the control terminal of the secondsynchronous rectifier, a second transistor coupled to the second diode,and an auxiliary circuit coupled first transistor for adjusting aswitching time of the second synchronous rectifier based on a controlsignal for the first synchronous rectifier.

Further areas of applicability will become apparent from the descriptionprovided herein. It should be understood that the description andspecific examples are intended for purposes of illustration only and arenot intended to limit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustration purposes only and arenot intended to limit the scope of the present disclosure in any way.

FIG. 1 illustrates a schematic view of a MOSFET controller according tothe Prior Art.

FIG. 2 illustrates a schematic view of a MOSFET controller includingbipolar junction transistors according to the Prior Art.

FIG. 3 illustrates a schematic view of a MOSFET controller includingbipolar junction transistors with matched orientations according to thePrior Art.

FIG. 4 illustrates a schematic view of a control circuit for controllinga switching device according to the present disclosure.

FIG. 5 illustrates a schematic view of a control circuit including adual transistor package.

FIG. 6 illustrates a schematic view of a 4-pin integrated circuitincluding a control circuit.

FIG. 7 illustrates a waveform of voltages simulated across terminals ofthe switching device of FIG. 5.

FIGS. 8A-B illustrate waveforms of turn ON and turn OFF switching timesof the switching device of FIG. 5 at various temperatures.

FIG. 9 illustrates a schematic view of a control circuit including atotem pole circuit.

FIG. 10 illustrates waveforms of switching times of the switching deviceof FIG. 9 with and without the totem pole circuit.

FIG. 11 illustrates a schematic view of a control circuit includingunmatched transistors.

FIG. 12 illustrates a schematic view of a control circuit including aswitching device and means for adjusting a switching time of theswitching device.

FIG. 13 illustrates a schematic view of a control circuit including aswitching device and means for adjusting a switching time of theswitching device.

FIG. 14 illustrates a schematic view of a control circuit including aresistor coupled to a bias voltage source.

FIG. 15 illustrates a synchronous rectifier with a control circuitaccording to the present disclosure.

FIG. 16 illustrates a synchronous rectifier with a control circuitincluding a Schottky diode.

FIGS. 17A-B illustrate waveforms of simulated turn ON switching times ofthe switching device of FIG. 16 with and without the Schottky diode.

FIGS. 18A-B illustrate waveforms of measured turn ON switching times ofthe switching device of FIG. 16 with and without the Schottky diode.

FIG. 19 illustrates a synchronous rectifier with a 4-pin integratedcontrol circuit.

FIG. 20 illustrates a synchronous rectifier including a control circuitwith unmatched transistors.

FIG. 21 illustrates a synchronous rectifier with a control circuitincluding transistors and a Schottky diode.

FIG. 22 illustrates a synchronous rectifier including a control circuitwith means for adjusting a switching time including two diodes.

FIGS. 23A-B illustrate a power supply and a synchronous rectifierincluding a control circuit with an auxiliary circuit according to thepresent disclosure.

FIGS. 24A-D illustrate waveforms simulated turn ON and OFF switchingtimes of the synchronous rectifier of FIG. 23.

FIGS. 25A-B illustrate waveforms of measured turn ON and OFF switchingtimes of the synchronous rectifier of FIG. 23 under at full loadcondition.

FIGS. 26A-D illustrate waveforms of measured turn ON and OFF switchingtimes of the synchronous rectifier of FIG. 23 under various loadconditions.

FIGS. 27A-B illustrate a schematic view of a power supply and integratedcircuit including a control circuit with a Darlington circuit.

FIG. 28 illustrates a schematic view of an integrated circuit includinga control circuit with an auxiliary circuit.

FIGS. 29A-B illustrate a schematic view of a power supply and a dualcathode integrated circuit including two switching devices and twocontrol circuits.

FIG. 30 illustrates a schematic view of a power supply with a push-pullconverter topology.

FIGS. 31A-B illustrate a schematic view of a power supply and controlcircuit with a dual totem pole circuit.

FIG. 32 illustrates a schematic view of a control circuit with means foradjusting a switching time of the switching device.

FIG. 33 illustrates a schematic view of a control circuit with anauxiliary circuit.

FIG. 34 illustrates a schematic view of a control circuit with unmatchedtransistors.

FIG. 35 illustrates a schematic view of a control circuit with a Bakerclamp circuit.

FIG. 36 illustrates a schematic view of a simplified control circuit.

FIG. 37 illustrates a schematic view of a full-bridge rectifieraccording to the present disclosure.

FIGS. 38A-B illustrate waveforms of simulated switching times ofswitching devices included in FIG. 37.

FIGS. 39A-B illustrate waveforms of measured switching times ofswitching devices included in FIG. 37.

FIG. 40 illustrates a schematic view of a full-bridge rectifierincluding totem pole circuit.

FIG. 41 illustrates a schematic view of a full-bridge rectifier withunmatched transistors.

FIG. 42 illustrates a schematic view of a full-bridge rectifier withunmatched transistors and a totem pole circuit.

FIG. 43 illustrates a schematic view of an integrated circuit includinga full-bridge rectifier according to the present disclosure.

FIGS. 44A-B illustrate schematic views of a power supply and anintegrated circuit including a full bridge rectifier.

FIG. 45 illustrates a 3-pin integrated circuit including a controlcircuit with a totem pole circuit.

FIG. 46 illustrates a block diagram of a multi-stage power supplyincluding switching devices and control circuits.

DETAILED DESCRIPTION

The following description is merely exemplary in nature and is notintended to limit the present disclosure, application, or uses.

A control circuit according to one embodiment of the present disclosureis illustrated in FIG. 4 and indicated generally by reference number400. The control circuit 400 includes a transistor Q1, a transistor Q2,a diode D1, a diode D2, and a switching device Q. The switching device Qhas a control terminal, a drain terminal and a source terminal. Theswitching device Q includes an intrinsic diode D. The transistor Q1 iscoupled to the source terminal of the switching device Q, and thetransistor Q2 is coupled to the drain terminal of the switching deviceQ. The transistor Q1 is also coupled to the control terminal of theswitching device Q. The control circuit 400 further includes resistorsR1, R2 coupled to transistors Q1, Q2, respectively.

The control circuit 400 is configured to allow current flow in only onedirection through the switching device Q. The configuration also permitsthermal tracking of the transistors Q1, Q2 sufficient for reliable andstable emulation of an ideal diode over a temperature range includingextreme temperatures. The inclusion of diodes D1, D2 further enhancesreverse voltage ratings for transistors Q1, Q2. And, the control circuitof the present disclosure may provide low cost and low component countcontrol circuits, which provide cost savings in parts and assembly.

In use, the control circuit 400 holds the switching device Q off when avoltage at node A is less than a voltage at node B. Under thiscondition, the intrinsic diode D of the switching device Q will bereverse biased. As shown in FIG. 4, the transistor Q2 is diodeconnected. With reference to a bipolar transistor having a collectorterminal and a base terminal, a diode connected transistor includes thecollector terminal being connected to the base terminal. The diode D2and diode-connected transistor Q2 are reverse biased. A bias voltage ispresent at the bias voltage input terminal and providing bias to thetransistor Q1 via resistor R2. The bias voltage holds transistor Q1 ON,which in turn, holds the switching device Q OFF. Thus, the switchingdevice Q blocks the flow of current from node B to node A. While theswitching device Q is illustrated as a FET, it should be appreciatedthat a different type of electrical, electromagnetic orelectromechanical switching device can be employed in other embodiments(e.g. power MOSFET, JFET, bipolar transistor, BJT, IGBT, etc.). Further,the switching device Q and other transistors disclosed herein may beeither n-channel or p-channel.

When the voltage at node A exceeds the voltage at node B, the intrinsicdiode D of the switching device Q starts to become forward biased,allowing current to flow from node A to node B. At the same time, diodeD2 and diode-connected transistor Q2 start to become forward biased. Thecurrent flow through diode D2 and transistor Q2 begins to steal currentfrom transistor Q1. This, in turn, begins to turn the transistor Q1 OFF,which increases the voltage at the control terminal of the switchingdevice Q. The voltage at the control terminal of the switching device Qcontinues to increase toward a threshold voltage of the switching deviceQ. At some point, the switching device Q starts to turn ON. As currentflow from node A to node B increases through the switching device Q,transistor Q1 (used as a common emitter amplifier with diode D1)attempts to decrease current through transistor Q1 and diode D1. As thecurrent through transistor Q1 decreases, the voltage drop acrosstransistor Q1 increases. The increase voltage drop increases the voltageat the control terminal of the switching device Q linearly. At somepoint, on-resistance of the switching device Q becomes dominant. Thevoltage drop across transistor Q1 increases until the transistor Q1 isturned OFF, which holds the switching device Q ON. When the switchingdevice Q is ON, current flows from node A to node B.

Although not denoted in FIG. 4, resistors R1, R2 have the sameresistance value. It should be appreciated that the resistance values ofthe resistors may be different in other embodiments to adjust aswitching time of the control circuit. Also, as illustrated in FIG. 4,transistors Q1, Q2 are bipolar transistors. In other embodiments,various types of transistors and equivalent switching devices may beemployed as transistor Q1 and/or transistor Q2 for various reasons, suchas cost and operating characteristics. Further, other diode devices orequivalents may be included as diode D1 and/or diode D2 in still otherembodiments. For example, a zener diode, Schottky diode, tunnel diode,or silicon controlled rectifier may be employed.

The control circuit 400 may be used in conjunction with various types ofswitching devices, such as an input switching device, and outputswitching device, or a converter switching device. Specifically, aswitching device may be included in a polarity protection FET circuit,an ORing FET circuit, or a power rectification circuit of a powersupply, such as a synchronous rectifier or an active bridge rectifier.The examples listed herein are not intended to be exhaustive. Instead,it should be understood that the present disclosure can be applied to anumber of different switching devices commonly used in electronics,electronic devices, electromagnetic devices and electromechanicaldevices.

FIG. 5 illustrates another embodiment of a control circuit 500 forcontrolling an ORing FET Q having a drain terminal, a source terminal,and a gate terminal. The ORing FET Q and control circuit 500 may beemployed on an output of a power supply. The control circuit 500 allowscurrent flow in only one direction through the ORing FET Q to ensurecurrent flow from the power supply and not into the power supply. Thecontrol circuit 500 includes a transistor Q1, a transistor Q2, a diodeD1, and a diode D2. The transistors Q1, Q2 are bipolar junctiontransistors (BJTs). Each BJT has a collector terminal, a base terminal,and an emitter terminal. The base terminal of the transistor Q1 iscoupled to the base terminal of the transistor Q2. The emitter of thetransistor Q1 is coupled to diode D1, which is coupled to the sourceterminal of the ORing FET Q. The emitter of the transistor Q2 is coupledto diode D2, which is coupled to the drain terminal of the ORing FET Q.The gate terminal of the FET Q is coupled to the collector terminal ofthe transistor Q1. Transistor Q2 is diode connected. The control circuit500 also includes resistors R1, R2. Each of resistor R1, R2 has aresistance of 4.7 kΩ. It should be appreciated that one or moreresistors included in other embodiments of the present disclosure mayinclude a greater or lesser resistance.

As shown, transistors Q1, Q2 are packaged together in a BCM61 integratedcircuit. The packaging of the transistors Q1, Q2 improves the thermaltracking of the transistors. The BCM61 integrated circuit is anoff-the-shelf, generic component. Other known dual transistor integratedcircuits may be employed in other embodiments of the present disclosure.For example, a BC847, a BC847BS, a FFB3904, a PUMX1, a BC847BD, a BCV61,a BCM846S, a ZXTD09N50 or a BCM847BS integrated circuit may be includedin other embodiments and/or applications of a control circuit accordingto the present disclosure. As further shown in FIG. 5, diodes D1, D2 arepackaged together in BAV23 integrated circuit. The packaging of thediodes D1, D2 improves the thermal tracking of the diodes. The BAV23integrated circuit is also an off-the-shelf, generic component. Otherdual diode integrated circuits, such as a BAS28, a BAV70, a BAS70-07 ora BAW101, may be employed in other embodiments of a control circuitaccording to the present disclosure. Further, as shown in FIG. 6,transistors Q1, Q2 and diodes D1, D2 can be employed in a 4-pinintegrated circuit 600. The 4-pin integrated circuit may be anintegrated circuit customized to the particular application or ageneric, off-the-shelf component. Although each of these differentpackaging options is disclosed relative to the ORing FET application, itshould be understood that the packaging options are equally applicableto the various applications for which a control circuit of the presentdisclosure is suited.

Referring again to FIG. 5, as referenced above, diodes D1, D2 areconfigured to block voltage to the transistors Q1, Q2. Diode D2 providesprotection to the base-emitter junction of the transistor Q2 from avoltage output and/or a load voltage. By protecting the base-emitterjunction in this manner, the control circuit 500 can be employed in anapplication with a voltage output up to about 205V, the controlcircuit's reverse voltage rating. A reverse voltage rating indicateswhat voltage can be present at the drain terminal of the ORing FET Qwithout damaging components included in the control circuit (V_(r) ofthe diode D2+V_(eb) of the transistor Q2). In other embodiments, areverse voltage rating can be altered by including one or more differentdiodes in a control circuit. For example, a 1N4007 diode can provide areverse voltage rating up to about 1000V. In another example, a BAW101dual diode, integrated circuit can provide a reverse voltage rating upto about 300V.

The control circuit 500 is coupled to the source of the ORing FET Q viadiode D1. In some applications of the control circuit 500, the diode D1is included merely to provide a matching offset for diode D2. In otherapplications, such as a polarity protection FET, diode D1 can provide asimilarly enhanced reverse voltage rating to transistor Q1 for highinput voltage conditions.

FIG. 7 illustrates waveform simulations of the control circuit 500. Thewaveform simulations show voltages measured at the gate of the ORing FETQ (bottom) and across the source and drain terminals of the ORing FET Q(top). The gate voltage is proportional to the source to drain voltagewhen Q1 is in linear mode. The higher the source to drain voltage drop,the higher the gate voltage of the ORing FET Q. The linear mode allowsthe ORing FET Q to respond to the output current through the ORing FETQ, rather than simply switching between an ON state and an OFF state.Additional waveform simulations are illustrated in FIGS. 8A-B. Thewaveform of FIG. 8A illustrates voltage at the gate of the ORing FET Qduring a transition from OFF to ON at temperatures 0° C., 50° C., and100° C. The waveform of FIG. 8B illustrates voltage at the gate of theORing FET Q during a transition from ON to OFF at temperatures 0° C.,50° C., and 100° C. As shown, variations in the temperature have minimaleffect on the control circuit 500 turning the ORing FET Q ON and OFFeven at extreme temperatures.

Control circuits according to the present disclosure allow current toflow in only one direction by controlling switching of a switchingdevice, essentially emulating an ideal diode. The operatingcharacteristics and the simplicity of the control circuit describedherein provide for a wide variety of applications of the controlcircuits. Across the variety of applications, operating parameters ofcontrol circuits can be adapted to the conditions of a specificapplication. In particular, turn ON and/or OFF switching time of aswitching device coupled to a control circuit may be critical in someapplications. FIG. 9 illustrates a control circuit 900 coupled to aswitching device Q having a control terminal, a drain terminal and asource terminal.

The control circuit 900 includes two transistors Q1, Q2 packagedtogether and two diodes D1, D2 packaged together. The control circuit900 also includes means for adjusting a switching time of the switchingdevice Q. In this embodiment, the means for adjusting includes a totempole circuit 902. The totem pole circuit 902 includes transistors Q4,Q5. The transistors Q4, Q5 are coupled in series between a bias voltagesource and the source terminal of the switching device Q. And, thetransistors Q4, Q5 are coupled to the control terminal of switchingdevice Q. As shown in simulated waveforms included in FIG. 10, the totempole circuit 902 provides turn ON and OFF switching times faster thanthe control circuit 500. Waveform 1002 illustrates the switching timewithout a totem pole circuit, and waveform 1004 illustrates theswitching time of the control circuit 500 with totem pole circuit 902.For waveform 1002, the rise time is about 51.68 μs and the fall time isabout 47.56 μs. For waveform 1004, the rise time is about 940.4 ns andthe fall time is about 622.2 ns. It should be appreciated that differentconfiguration of a control circuit may be employed in conjunction withmeans for adjusting the switching time of a switching device, such as atotem pole circuit. For example, as shown in FIG. 11, a control circuit1100 is coupled to a switching device Q and includes a totem polecircuit 1102. As compared to the control circuit 900, the orientation ofthe transistor Q2 is changed such that the transistors Q1, Q2 areunmatched. The unmatched transistor Q1, Q2 provide for increase reversevoltage rating for the control circuit 1100.

FIGS. 12-14 disclose other means for adjusting a switching time of aswitching device Q. FIG. 12 illustrates a control circuit 1200 coupledto a switching device Q having a control terminal. The control circuit1200 includes a transistor Q5 and a diode D3. The transistor Q5 is abipolar junction transistor (BJT) having a base terminal, a collectorterminal, and an emitter terminal. The diode D3 is coupled between thebase terminal and the emitter terminal of the transistor Q5. The turn ONtime of the control circuit 1200 is comparable to the turn ON switchingtime of the control circuit 500. The control circuit 1200 providesfaster turn OFF of the switching device Q relative to the controlcircuit 500. The turn OFF switching time of the control circuit 1200 iscomparable to the turn OFF switching time of the control circuit 900.

FIG. 13 illustrates a control circuit 1300 coupled to a switching deviceQ having a control terminal. The control circuit includes a transistorQ5 and a resistor R3. The transistor Q5 is a BJT having a base terminal,a collector terminal and an emitter terminal. The control circuit 1300provides faster turn ON of the switching device Q relative to thecontrol circuit 500, but not as fast as the turn ON switching timeprovided by the control circuit 900. The turn ON switching can beadjusted further by changing the resistance of the resistors R3. Asshown, the resistor R3 is 1 kΩ. The turn OFF switching time of thecontrol circuit 1200 is comparable to the turn OFF switching time of thecontrol circuit 900.

FIG. 14 illustrates control circuit 1400 coupled to a switching deviceQ. The control circuit 1400 includes a diode D3, a resistor R3, and atransistor Q5 having a base terminal, a collector terminal and anemitter terminal. The diode D3 is coupled between a bias voltage sourceand the emitter of transistor Q5. The resistor R3 is coupled between thecontrol circuit 1400 and a voltage source 1402. In this particularembodiment, the bias voltage source tracts the ON and OFF of theswitching device Q. When the switching device Q is OFF, the voltageafter R3 is clamped to zero via the diode D3 and the transistor Q5.Accordingly, a current flows through the resistor R3, which may berequired to be a high rated resistor. In this particular example, theresistor R3 is a 1 kΩ, 0.25 W resistor. The control circuit 1400provides faster turn ON and OFF switching times relative to the controlcircuit 500. The control circuit 1400 provides turn OFF switching timescomparable to the control circuit 900. In other embodiments of thecontrol circuits illustrated in FIGS. 9 and 11-14, a resistor isconnected between the means for adjusting the switching time and thecontrol terminal of switching device Q.

While each of the control circuits disclosed in FIGS. 9 and 11-14 areillustrated as controlling an ORing FET, it should be appreciated thatone or more of the disclosed control circuits can be employed in otherapplications, such as a synchronous rectifier. As illustrated in FIG.15, a control circuit 1500 is coupled to a synchronous rectifier Q forcontrolling the synchronous rectifier Q having a control terminal, adrain terminal, and a source terminal. The control circuit 1500 includesa totem pole circuit 1502, which is consistent with totem pole circuit1002. As explained above, the totem pole circuit 1502 is included asmeans for adjusting a switching time of the synchronous rectifier Q.

FIG. 16 illustrates a control circuit 1600 coupled to a synchronousrectifier Q for controlling the synchronous rectifier Q having a controlterminal, a drain terminal, and a source terminal. The control circuit1600 includes transistors Q1, Q2 and resistors R1, R2. Each of thetransistors Q1, Q2 has a base terminal, a collector terminal, and anemitter terminal. Transistor Q1 is configured as a common emitteramplifier with a diode D1 to enhance the reverse voltage rating oftransistor Q1. Transistor Q2 is diode connected to offset thebase-emitter voltage of transistor Q1. A diode D2 is coupled totransistor Q2 to enhance the reverse voltage rating of transistor Q2 andprovide matching offset for diode D1.

The control circuit 1600 also includes means for adjusting the switchingtime of the synchronous rectifier Q. The means for adjusting theswitching time includes a totem pole circuit 1602 and a diode D3 coupledbetween the base terminal and the collector terminal of transistor Q1.The Schottky diode D3 adjusts the switching time of the synchronousrectifier Q by limiting the saturation of transistor Q1. When thesynchronous rectifier Q is OFF, transistor Q1 is saturated (ON). For thesynchronous rectifier Q to be turned ON, the transistor Q1 needs totransition from ON to OFF. When the transistor Q1 is substantiallysaturated, the transition of the transistor Q1 from ON to OFF occursover a period of time. By including the Schottky diode D3, thesaturation of the transistor Q1 is limited. In this embodiment, thesaturation of transistor Q1 is limited to about 0.4V. Without diode D3,the saturation of the transistor Q3 is about 0.02V. Therefore, theSchottky diode D3 shortens the period of time for transitioning thetransistor Q1 out of saturation and from ON to OFF. While, the diode D3is included in addition to the totem pole circuit 1602 in controlcircuit 1600, it should be appreciated that diode D3 can be includedwith or without further means for adjusting a switching time of aswitching device in other embodiments of the present disclosure.Further, while the diode D3 is illustrated as a Schottky diode (BAT54),it should be appreciated that other type of diode and packaging can beemployed in other embodiments of the present disclosure. For example,diode D3 can be a different off-the-shelf component, such as a TBAT54, aBAT54CW, a BAT54C, a BAT54A, etc.

The faster switching time of the control circuit 1600, as compared tothe control circuit of FIG. 15, is illustrated by comparison of FIGS.17A-B. FIG. 17A shows the turn ON switching time of the control circuit1600 without the Schottky diode D3. The switching time is about 564nanoseconds. The FIG. 17B shows the turn ON switching time of thecontrol circuit 1600 with the Schottky diode D3. The switching time isreduced to about 63 nanoseconds. Additionally, FIGS. 18A and 18Billustrate the adjustment of the switching time provided by diode D3measured by an oscilloscope. FIG. 18A shows the switching time of thesynchronous rectifier Q illustrated in FIG. 15. Channel 2 is voltage tothe control terminal of the synchronous rectifier Q of FIG. 15, andchannel 3 is voltage between the drain terminal and the source terminalof the synchronous rectifier Q of FIG. 15. Similarly, FIG. 18B shows theswitching time of the synchronous rectifier Q illustrated in FIG. 16.Channel 2 is voltage to the control terminal of the synchronousrectifier Q of FIG. 16, and channel 3 is voltage between the drainterminal and the source terminal of the synchronous rectifier Q of FIG.16. Channel 4 is the source current of the respective synchronousrectifier. A comparison of FIGS. 18A and 18B illustrates that the turnON delay is substantially reduced by including the Schottky diode D3.

FIG. 19 illustrates an alternate embodiment of the control circuit 1600.As shown in FIG. 19, transistors Q1, Q2, diodes D1, D2, and Schottkydiode D3 are included in a 4-pin integrated circuit 1900. The 4-pinintegrated circuit 1900 provides for improved thermal tracking andprinted circuit board (PCB) space savings. In other embodiments, anintegrated circuit may include resistors R1, R2, a switching device Qand/or other means for adjusting a switching time of the switchingdevice Q.

FIG. 20 illustrates a control circuit 2000 according to anotherembodiment of the present disclosure. The control circuit 2000 isemployed in a power supply to control a synchronous rectifier Q. Thecontrol circuit includes transistors Q1, Q2 and diodes D1, D2. Eachtransistor Q1, Q2 is a BJT having a base terminal, a collector terminal,and an emitter terminal. The emitter terminal of transistor Q1 iscoupled to diode D1, and the collector terminal of transistor Q2 iscoupled to diode D2. By changing the orientation of transistor Q2, adegree of balance and symmetry is lost in the control circuit 1600. Thechange in the orientation of transistor Q2, however, takes advantage ofthe base-collector junction of transistor Q2. The unmatched orientationof transistor Q2 allows the control circuit 2000 to have a reversevoltage rating as high as about 365V. While the unmatched orientation ofthe transistor Q1, Q2 is disclosed relative to synchronous rectifier Q,it should be appreciated that unmatched transistors can be included inother application of a control circuit of the present disclosure toenhance the reverse voltage rating and reduce control circuit cost.

FIG. 21 illustrates a control circuit 2100 coupled to a synchronousrectifier Q for controlling the synchronous rectifier Q. The controlcircuit 2100 includes transistors Q1, Q2, a diode D3 and a totem polecircuit 2102. Diodes D1, D2 are absent from the prior control circuits.The control circuit 2100 is suitable for various applications in whichthe transistors Q1, Q2 provide sufficient reverse voltage ratings.Another embodiment of a control circuit 2200 according to the presentdisclosure is illustrated in FIG. 22. As shown, control circuit 2200includes a diode D5 in place of transistor Q5 (as shown in FIG. 16). Byincluding Schottky diode D5 with a common cathode connection withSchottky diode D3, a dual Schottky diode, common-cathode integratedcircuit (e.g., BAT54C) can be used to implement the control circuit2200.

While each of the synchronous rectifiers illustrated in FIGS. 16 and19-22 are included in a discontinuous conduction mode (DCM) flybackpower converter, it may also be included in a continuous conduction mode(CCM)+DCM flyback converter in other embodiments. In still otherembodiments, a control circuit disclosed herein can be employed in anumber of different types of power converters, power inverters, andpower supplies. For example, a control circuit and a switching devicemay be included in several different types of converters, such as aflyback converter, a forward converter, a buck converter, a boostconverter, a buck/boost converter, a Cuk converter, a sepic converter, azeta converter, a push-pull converter, a half bridge converter, a fullbridge, a resonant converter, a bridge rectifier, etc.

FIGS. 23A-B illustrate a power supply 2300 including two synchronousrectifiers Qa, Qb and a control circuit 2302 for controlling thesynchronous rectifier Qb. FIG. 23B illustrates the power supply 2300including a transformer T1, an inductor L, and an output capacitor C.Based on the configuration of the power supply, the synchronousrectifier Qb is a freewheeling synchronous rectifier. The controlcircuit 2302 is intended to couple the power supply 2300 at thedesignated nodes. The control circuit 2302 includes transistors Q1, Q2included in a BCM61 package and resistors R1, R2, which have the sameresistance (1.2 kΩ). The control circuit also includes diodes D1, D2.The synchronous rectifier Qb includes an intrinsic diode Db. Theintrinsic diode Db may be packaging together with or separately from thesynchronous rectifier Qb. As shown in FIG. 23A, the control circuit 2302includes resistors R1, R2 and means for adjusting the switching time ofthe synchronous rectifier Qb. The means for adjusting switching ON timeincludes a Schottky diode D3 and a cascaded emitter follower drivercircuit including transistors Q3, Q4, Q5 and diode D5. Although notshown, diodes D3, D5 are included in a BAT54C package. Transistors Q3,Q4 are included in a BC817 package, and transistor Q5 is included in aBC807 package. In this particular embodiment, the output of the powersupply is 12.0V at 25.0 amps. While each of the packages included in thecontrol circuit 2302 is a generic, off-the-shelf package, it should beunderstood that each of the particular components included in thecontrol circuit of FIG. 23A can be packaged differently, generic orcustom, in other embodiments of the present disclosure.

Simulated waveforms for the control circuit 2302 are illustrated in FIG.24A-D. FIG. 24A illustrates the turn ON switching time of the switchingdevice Qb with only the Schottky diode D3 and the totem pole circuitincluding transistors Q4, Q5. The turn ON switching time of theswitching device Qb is about 75 ns. FIG. 24B illustrates the turn ONswitching time of the switching device Qb with the Schottky diode D3,the Darlington circuit and the totem pole circuit. The turn ON switchingtime of the switching device Qb is improved to about 30 ns. A turn ONswitching time of about 30 ns may make the control circuit 2302particularly suited for applications with very high switchingfrequencies, e.g., 400 kHz or above, which reduce conduction through anintrinsic diode included in synchronous rectifier Qb.

Referring again to FIG. 23A, the means for adjusting switching OFF timealso includes an auxiliary circuit 2304. The auxiliary circuit 2304includes a transistor Q6, resistors R3, R4, and a capacitor C1. Theauxiliary circuit 2304 is controlled by a control signal of thesynchronous rectifier Qa, shown as pulse wide modulated (PWM) signal.The PWM signal originates from a PWM controller coupled to the powersupply 2300 (not shown). In use, an ON-time of synchronous rectifier Qacompliments an ON-time of the synchronous rectifier Qb. When synchronousrectifier Qa is turned ON, synchronous rectifier Qb is turned OFF. Whensynchronous rectifier Qb fails to turn OFF fast enough (i.e., bothsynchronous rectifiers Qa, Qb are ON), a shoot through current conditionexists. The shoot through current condition can cause inefficiency andeven failure of one or both of the synchronous rectifiers Qa, Qb. Theauxiliary circuit 2304 reduces the occurrence of the shoot throughcondition. When the PWM goes high to control the synchronous rectifierQa ON, the PWM also drives transistor Q6 ON. When transistor Q6 turnsON, the collector terminal of transistor Q1 is clamped to V_TRN, whichturns the transistor Q3 OFF, which causes the synchronous rectifier Qbto turn OFF.

The auxiliary circuit 2304 adjusts turn OFF switching time of thesynchronous rectifier Qb. The exemplary implementation of the auxiliarycircuit 2304 includes resistor R3 being 100Ω, the capacitor C1 being 100pF, and the transistor Q6 being 2N7002. The synchronous rectifier Qb isa FDP060AN08A0 device. The turn OFF switching time is illustrated insimulated waveforms of FIGS. 24C-D. FIG. 24C illustrates the turn OFFswitching time of the switching device Qb without the auxiliary circuit2304. The turn OFF switching time of the switching device Qb is about37.1 ns. FIG. 24D illustrates the turn OFF switching time of theswitching device Qb with the auxiliary circuit 2304. The turn OFFswitching time of the switching device Qb is improved to about 7.6 ns.The turn OFF switching time can be programmed and/or optimized byvarying the values of resistors R3, R4 and capacitor C1 in the auxiliarycircuit 2304. The turn OFF switching time of the switching device Qb canalso be programmed on-time propagation delay of the included PWMcontroller. While resistors R3,R4 and capacitor C1 are included toadjust a switching time, it should be appreciated that one or more ofresistors R3, R4 and capacitor C1 may be included or excluded in otherembodiments depending on the particular application of the embodiment, avoltage level of a PWM signal and timing of a PWM signal. For example,in at least one embodiment, a PWM signal may be connected directly to agate terminal of a transistor Q6.

The power supply disclosed above in FIG. 23B may be included in an AC/DCpower supply application. The AC input to the power supply can rangefrom about 90-264 volts (AC) at about 47-63 Hz with an output of about12.0 volts at about 20.0 amps, about 5.0 volts at about 35.0 amps, about3.3 volts at about 15.0 amps, and about 5.0 volts stand-by at about 2.0amps. Switching frequency of the two synchronous rectifiers included inthe forward converter is about 125 kHz. As understood by the particularoutputs of the power supply, one implementation of the power supplydescribed above is in a personal computer, such as a laptop. It shouldbe understood that the control circuit can be employed in a number ofdifferent power supplies and power supply sub-assemblies to providepolarity protection, voltage conditioning and/or voltage output.

FIGS. 25A-B further illustrate actual measured waveforms for thedrain-source voltage of the synchronous rectifier Qb at channel 1 andthe gate voltage of the synchronous rectifier Qb at channel 2. Awaveform associated at channel 3 is the PWM signal provided to thetransistor Q6. As shown, the turn ON switching time of the synchronousrectifier Qb is about 44 ns. The leading turn OFF switching time of thesynchronous rectifier Qb is about 102 ns, which is programmable anddependent on the auxiliary circuit, specifically the values of theresistors R3 and capacitor C1. FIGS. 26A-D further illustrate waveformsof the switching times of the synchronous rectifier Qb as controlled bythe control circuit 2302. The various waveforms show the switching timesof the synchronous rectifier Qb under various loading conditions. FIG.26A illustrates the switching times of the synchronous rectifier Qb forabout 5.0 volts at about 0.5 amps. FIG. 26B illustrates the switchingtimes of the synchronous rectifier Qb for about 5.0 volts at about 1.0amp. FIG. 26C illustrates the switching times of the synchronousrectifier Qb for about 5.0 volts at about 10.0 amps. FIG. 26Dillustrates the switching times of the synchronous rectifier Qb forabout 5.0 volts at about 25.0 amps. Spikes in the gate voltage of thesynchronous rectifier Qb are visible in the lower current waveforms dueto current through the intrinsic diode Db of the synchronous rectifierQb and then the ON resistance is dominant.

As explained above, the packaging of the various components of a controlcircuit can provide increased cost saving, reduced PCB space, andimproved thermal tracking. FIG. 27A illustrates a 3-pin integratedcircuit 2700 including a control circuit and a synchronous rectifier Q.FIG. 27B illustrates a power supply 2702 application for the 3-pinintegrated circuit 2700. The 3-pin integrated circuit can be distributedas a smart diode. It should be appreciated that an integrated circuitcan include more or less components to provide increased or decreasedflexibility in a particular embodiment. For example, a totem pole, aSchottky diode, a Darlington circuit, a Baker clamp circuit, or anauxiliary circuit can be excluded from the integrated circuit toincrease user control of the various components of these means foradjusting the switching time of the synchronous rectifier Q. Forexample, FIG. 28 illustrates an exemplary embodiment including a 4-pinintegrated circuit 2800, which incorporates a Schottky diode D3, aDarlington circuit, a totem pole circuit, and an auxiliary circuit. TheSchottky diode D3, Darlington circuit, totem pole circuit, and auxiliarycircuit are each included to adjust a switching time of switching deviceQ. In other embodiments, a synchronous rectifier Q can be excluded froman integrated circuit to provide a user with a choice regarding whichswitching device to employ.

FIGS. 29A-B illustrate yet another embodiment of the present disclosure.FIG. 29A shows an integrated circuit 2900 including switching devicesQa, Qb and control circuits 2902, 2904. Each of the switching devicesQa, Qb includes a gate terminal, a drain terminal, and a sourceterminal. Control circuit 2902 control switching device Qa, and controlcircuit 2904 control switching device Qb. Each control circuit includesa Darlington circuit, a totem pole circuit, and a Schottky diode D3. Inthis particular embodiment, the source of the switching device Qa andthe source of the switching device Qb are coupled to a common anode pinof the integrated circuit. The integrated circuit 2900 includes acathode_A pin and a cathode_B pin, resulting in a dual cathodeintegrated circuit. Thus, the integrated circuit 2900 provided efficientconnection to a forward converter power supply 2906. It should beappreciated that a different number of switching devices and associatedcontrol circuits can be included in other embodiments of the presentdisclosure. It should also be appreciated that the particular couplingof a switching device and/or a control circuit to one or more pins of anintegrated circuit can be altered, depending on a particular applicationof an embodiment. FIG. 30 illustrate a push-pull converter 3000. Thepush-pull converter 3000 illustrates another embodiment in which thedual cathode integrated circuit 2900 may be employed.

FIGS. 31A-B illustrate a power supply 3100 and control circuit 3102. Thepower supply includes switching devices Qa, Qb. The control circuit 3102includes a dual totem pole circuit 3104 (transistors Q3, Q7 being afirst stage totem pole circuit, and transistors, Q4 Q5 being a secondstage totem pole circuit), an auxiliary circuit 3106, and a diode D3 foradjusting a switching time of the switching device Qb. It should beappreciated that a different combination of the means for adjusting aswitching time of a switching device can be included in otherembodiments of the present disclosure. For example, FIGS. 32-36illustrate several addition embodiments of a control circuit for powersupply 2300. Each embodiment includes one or more means for adjusting aswitching time of the switching device Qb of power supply 2300. FIG. 32illustrates a control circuit 3200 with an auxiliary circuit 3202 and aDarlington circuit 3204. Diode D5 is connected to an emitter oftransistor Q4. Diodes D3, D5 can be included in a dual diode, commoncathode integrated circuit. FIG. 33 illustrates a control circuit 3300including an auxiliary circuit 3302. The auxiliary circuit 3302 includestransistor Q6 having a control terminal, a capacitor C1, and resistorsR3, R4. Resistor R3 and capacitor C1 are coupled in series to a controlterminal of a bipolar transistor Q6. The auxiliary circuit 3302 clamps avoltage of an emitter terminal of transistor Q1 to nearly zero quicklydue to the differential circuit of C1 and R3.

FIG. 34 illustrates a control circuit 3400, which includes twotransistors Q1, Q2. Each of the transistors Q1, Q2 is a bipolar junctiontransistor having a base terminal, a collector terminal, and an emitterterminal. The transistor Q1 is coupled to a diode D1 via the emitterterminal, and the transistor Q2 is coupled to a diode D2 via thecollector terminal. The change in orientation, as compared to FIG. 23A,allows the base-collector junction of transistor Q2 to be utilized. Theunmatched orientation of Q2 allows the control circuit 3400 to have areverse voltage rating as high as about 365V.

FIG. 35 illustrate a control circuit 3500, which includes transistorsQ1, Q2, a totem pole circuit 3502, a Darlington circuit, a Baker clampcircuit, an auxiliary circuit 3504, resistors R1, R2, and diodes D1, D2,D3, D4, D5. Each of the transistors Q1, Q2 is a bipolar junctiontransistor having a base terminal, a collector terminal, and an emitterterminal. The base terminals of the transistors Q1, Q2 are coupled toone another and to the collector terminal of transistor Q2. A diode D4is coupled between the collector terminal of transistor Q2 and resistorR2. Diode D3 is coupled between the collector of transistor Q1 and thediode D4, creating collector-base junction of transistor Q1 coupled inparallel with diodes D3, D4. Diode D5 is coupled between the collectorof transistor Q1 and the totem pole circuit 3502. Selecting the properdiode D3, D4 can provide adjustment of the saturation limit of thetransistor Q1 and, in turn, adjustment of the switching time of thesynchronous rectifier Qb. Table 1 below shows different permutations ofdiode and Schottky diode for use as diodes D3, D4.

TABLE 1 D3 Diode Diode SBD SBD SBD D4 SBD Diode SBD Diode Short CircuitVb vs. Vc Vb = Vb = Vc Vb = Vc Vb = Vb = Vc + 0.4 Vc − 0.4 Vc + 0.3

As shown, each different combination of fast recovery diodes and/orSchottky diodes changes the saturation limit of the transistor Q1.Despite the inclusion of only fast recovery diodes and Schottky diodesin Table 1, it should be understood that a different type of diode canbe employed in other embodiments of a control circuit.

FIG. 36 illustrates a control circuit 3600, which includes a Darlingtoncircuit 3602 and an auxiliary circuit 3604. As shown, the Darlingtoncircuit 3602 includes fewer components than some of the control circuitsdescribed above. The control circuit 3600 provides a similar turn ONswitching time for the synchronous rectifier Qb to FIGS. 23A, 31A, and32-35. By including of the auxiliary circuit 3604, the control circuit3600 retains the programmable turn OFF switching time of the synchronousrectifier Qb. The switching device Qb, however, can not be turned offautomatically if the PWM signal is disabled, because the turn OFF reliesdirectly on transistor Q6 based on the omission of transistor Q5 ascompared to FIG. 23A.

According to another embodiment of the present disclosure, a full-bridgerectifier 3700 includes four switching devices Qa, Qb, Qc, Qd and fourcontrol circuits 3702, 3704, 3706, 3708. Each of the switching devicesis a RFP4332PBF. Each of the control circuits is coupled to a respectiveone of the switching devices. Control circuit 3704 is representative ofcontrol circuits 3702, 3706, 3708. Each control circuit includestransistors Q1, Q2 (ZXTD09N50DE6), diodes D1, D2 (DB1N4007). Eachcontrol circuit also includes resistors R1, R2, which each have aresistance of 10 kΩ. It should be appreciated that different type ofdiodes, transistors, resistor and switching devices can be employed inother embodiments of the present disclosure. The control circuit isconfigured to allow current flow in only one direction through theswitching device Qb.

FIG. 38A illustrates waveform simulations of the full-bridge rectifier3700 for an input voltage of 180 VAC at 50 Hz with a load resistor of10Ω. Waveforms 3802 illustrate voltages at control terminals ofswitching devices Qb, Qc. Waveforms 3804 illustrate voltages at controlterminals of switching devices Qa, Qd. Waveforms 3806 illustrate thesource current through Qa, Qb, and waveform 3808 illustrates the outputvoltage, of the full bridge rectifier 3700. FIG. 38B illustrates anexpanded view of a time interval of FIG. 38A. FIGS. 39A-B illustrate ameasured waveform from the full bridge rectifier 3700. The output fromthe full bridge rectifier 3700 may be up to about 3600 W. It should beappreciated that one or more of the components may be configured and/orchanged to alter an output voltage or current of a rectifier in adifferent embodiment.

As shown, control circuit 3704 includes a bias voltage Vb_bias. A biasvoltage is included in each of the control circuits 3702, 3706, 3708.The bias voltages for the control circuits 3702, 3704, 3706, 3708 can berectified from the AC input or using the bias voltage in the powersupply directly. A bias voltage PVCC, which is a bias voltage suppliedfrom a primary side within the power supply, may be used for controlcircuits 3702, 3704. The bias voltage for control circuits 3706, 3708can use other bias voltage rectified from an auxiliary winding of apower factor correction choke or divided by the bulk voltage. In otherembodiments, one or more different combinations of bias voltages may beapplied to control circuits 3702, 3704, 3706, 3708.

It should be appreciated that the control circuits included in the fullbridge rectifier 3700 can employ one or more means for adjusting theswitching time of a switching device. For example, as shown in FIG. 40,a totem pole circuit 4000 is included in a control circuit 4002 of afull bridge rectifier 4004 to adjust the switching time of switchingdevice Qb. In other embodiments, different combinations of theembodiments described above can be included in one or more of thecontrol circuits included in a full-bridge rectifier. For example, asshown in FIG. 41, the orientation of transistor Qb2 can be altered toaffect the reverse voltage rating of control circuit 4100. In anotherexample, as shown in FIG. 42, the orientation of the transistor Qb2 canbe altered in conjunction with a totem pole circuit included in acontrol circuit 4200. In still other embodiments according to thepresent disclosure, different contents and/or types of packaging may beemployed. As shown in FIG. 43, an integrated circuit 4300 can includefour switching devices and associated control circuits. The singleintegrated circuit includes six external pins for connecting to AC+,AC−, DC+, DC− and two bias voltages. The bias voltage can be providedfrom an AC input or from the power supply directly, as explained above.

FIG. 44A-B illustrates a power supply 4400 and integrated circuit 4402according to another embodiment of the present disclosure. The powersupply 4400 includes a transformer T1, an inductor L1, and a capacitorC1. The integrated circuit 4402 includes a full-bride rectifier withfour switching devices Qa, Qb, Qc, Qd. Each of the switching devices isconnected with one of four control circuits included in the integratedcircuit 4402. Each of the control circuits includes means for adjustinga switching time of the switching device connected thereto. It should beappreciated that, in other embodiments, each of the switching devicesand control circuits can be employed in separate packaging. For example,a rectifier can include four control circuits 4500 as shown in FIG. 45in combination with four FETs, packages separately. A number ofdifferent packaging options, generic and custom, are available forparticular applications of one or more control circuits and switchingdevices included in power supplied according to the present disclosure.

While several aspects of the present disclosure have been described withspecific reference to an ORing FET, a polarity protection FET, asynchronous rectifier, and/or an active bridge rectifier, it should beunderstood that each aspect of the present disclosure may be adapted toany one of the applications and/or embodiments described herein.

FIG. 46 illustrates a multi-stage power supply 4600 coupled to a load4602 according to another embodiment. The power supply includes a firststage and a second stage. The first stage includes a polarity protectionswitching device 4604 coupled to a control circuit 4606, a powerconverter 4608 with at least one switching device and a control circuit4610, and a ORing FET 4612 coupled to a control circuit 4614. The secondstage is substantially the same as the first stage. It should beappreciated that a different number of stages may be included in a powersupply of another embodiment. It should further be appreciated that adifferent number of switching devices may be coupled to a controlcircuit according to the present disclosure. Further, the powerconverter included in each illustrated stage can be any number of DC/DCor AC/DC topologies know to include at least one switching device.

Although several aspects of the present disclosure have been describedabove with reference to power supplies, it should be understood thatvarious aspects of the present disclosure are not limited to powersupplies, and can be applied to a variety of other switching devices andapplications.

By implementing any or all of the teachings described above, a number ofbenefits and advantages can be attained including improved systemreliability, reduced system down time, elimination or reduction ofredundant components or systems, avoiding unnecessary or prematurereplacement of components or systems, and a reduction in overall systemand operating costs.

What is claimed:
 1. A control circuit for controlling a rectifier havinga first terminal, a second terminal, and a control terminal, the controlcircuit comprising a first diode for coupling to the first terminal ofthe rectifier, a second diode for coupling to the second terminal of therectifier, a first transistor for coupling to the control terminal ofthe rectifier, a second transistor coupled to the second diode, and athird diode coupled between a first terminal of the first transistor anda second terminal of the first transistor to limit saturation of thefirst transistor, the first transistor coupled to the first diode, thecontrol circuit configured to allow current flow in only one directionbetween the first and second terminals of the rectifier.
 2. The circuitof claim 1 wherein the third diode is a Schottky diode.
 3. The circuitof claim 1 further comprising a bias voltage input terminal, a firstresistor coupled between the bias voltage input terminal and the firsttransistor, and a second resistor coupled between the bias voltage inputterminal and the second transistor.
 4. The circuit of claim 3 furthercomprising means for adjusting the switching time of the rectifier. 5.The circuit of claim 1 wherein the first transistor and secondtransistor are packaged together in a first integrated circuit.
 6. Thecircuit of claim 5 wherein the first diode and the second diode arepackaged together in a second integrated circuit.
 7. The circuit ofclaim 1 wherein the second transistor is diode-connected.
 8. The circuitof claim 7 wherein the second transistor is a bipolar junctiontransistor having a base terminal, an emitter terminal, and a collectorterminal, the emitter terminal of the second transistor being coupled tothe second diode.
 9. The circuit of claim 7 wherein the secondtransistor is a bipolar junction transistor having a base terminal, anemitter terminal, and a collector terminal, the collector terminal ofthe second transistor being coupled to the second diode.
 10. Anintegrated circuit comprising at least the control circuit of claim 1.11. A power supply system comprising at least one synchronous rectifierand the control circuit of claim 1 for controlling said synchronousrectifier.
 12. A power supply system comprising a bridge rectifierhaving a switching device and the control circuit of claim
 1. 13. Apower supply for supplying a current to an output, the power supplycomprising a rectifier having a first terminal, a second terminal, and acontrol terminal, and a control circuit coupled to the rectifier toallow current flow in only one direction through the rectifier, thecontrol circuit including a first transistor having a first terminal anda second terminal, a second transistor coupled to the second terminal ofthe rectifier, and a diode coupled between the first terminal of thefirst transistor and the second terminal of the first transistor tolimit saturation of the first transistor, the first transistor coupledto the first terminal of the rectifier and the control terminal of therectifier.
 14. The power supply of claim 13 wherein the control circuitfurther includes a first diode coupled between the first transistor andthe first terminal of the rectifier and a second diode coupled betweenthe second transistor and the second terminal of the rectifier.
 15. Thepower supply of claim 14 wherein the first diode and the second diodeare packaged together in a first integrated circuit.
 16. The powersupply of claim 15 wherein the first transistor and the secondtransistor are packaged together in a second integrated circuit.
 17. Thepower supply of claim 16 wherein the diode coupled between the first andsecond terminals of the first transistor is a Schottky diode.
 18. Thepower supply of claim 17 wherein the control circuit includes means foradjusting a switching time of the rectifier.
 19. The power supply ofclaim 18 wherein the means for adjusting the switching time includesmeans for adjusting an ON time of the rectifier.
 20. The power supply ofclaim 18 wherein the means for adjusting the switching time includesmeans for adjusting an OFF time of the rectifier.
 21. The power supplyof claim 13 wherein the rectifier is a synchronous rectifier.
 22. Thepower supply of claim 13 wherein the rectifier is a full bridgerectifier.
 23. The power supply of claim 14 wherein the secondtransistor is a bipolar junction transistor having a base terminal, anemitter terminal, and a collector terminal, wherein the collectorterminal is connected to the second diode.
 24. The power supply of claim14 wherein the second transistor is a bipolar junction transistor havinga base terminal, an emitter terminal, and a collector terminal, whereinthe emitter terminal is connected to the second diode.
 25. The powersupply of claim 13 further comprising a bias voltage source and whereinthe control circuit includes a first resistor coupled between the firsttransistor and the bias voltage source and a second resistor coupledbetween the second transistor and the bias voltage source.
 26. The powersupply of claim 13 wherein the first transistor and the secondtransistor are bipolar junction transistors each having a base terminal,an emitter terminal, and a collector terminal, the collector terminal ofthe second transistor is coupled to the second terminal of therectifier, the emitter terminal of the first transistor is coupled tothe first terminal of the rectifier, the diode is coupled between thecollector terminal of the first transistor and the base terminal of thefirst transistor to limit saturation of the first transistor.
 27. Thepower supply of claim 26 wherein the diode includes an anode and acathode, the cathode of the diode is directly coupled to the collectorterminal of the first transistor and the anode of the diode is directlycoupled to the base terminal of the first transistor, the collectorterminal of the second transistor is directly coupled to the secondterminal of the rectifier, the emitter terminal of the first transistoris directly coupled to the first terminal of the rectifier, the baseterminal of the first transistor is directly coupled to the baseterminal of the second transistor.